It is generally understood that, for a polymer blend to be useful as a coating material, outstanding performances with respect to several criteria including optical behaviour (e.g. gloss and weatherability), hardness, abrasion resistance etc. are required.
Vinylidene fluoride (VDF) polymers, possessing unique properties such as exceptional weathering resistance, good abrasion resistance, high mechanical strength and toughness, etc., have been used as base material for coatings, and more particularly for highly weatherable architectural coatings for years. For improving adhesion to substrates, pigment dispersion ability and increasing coating gloss, miscible VDF polymer blends with (meth)acrylic resins have been developed. As optical behaviour of a coating, including e.g. gloss, typically requires coating matrix continuity, thermodynamically miscible blends are preferred, especially for exterior architectural finish applications. Said miscibility also allows the final coating to maximize coating weatherability in long term exterior service.
In view of the above, blends of VDF polymers and (meth)acrylic resin at around a 70:30 weight ratio have been proven to provide optimum material performance with respect to criteria such as adhesion, toughness and optical clarity. However these said blends have reduced hardness and abrasion or mar resistance compared to PVDF alone.
Compositions based on VDF polymers and polyimide polymers comprising glutarimide or methacrylimide moieties have been also disclosed.
EP 0216505 A (ROHM AND HAASCOMPANY) 1 Apr. 1987 discloses polymethacrylimide/thermoplastic blends whereby the degree of imidization is ranging from preferably 50 to 85%. The disclosed blends generally exhibit a better combination of properties including better melt viscosity, greater compatibility and/or miscibility, greater weatherability, greater ductility and better solvent resistance when the degree of imidization is higher. Some examples of polymethacrylimide/Kynar® 720 polyvinylidene fluoride (PVDF) blends are demonstrated whereby the degree of imidization is at least 82%.
U.S. Pat. No. 5,284,913 (MITSUBISHI RAYON CO) 21 Jan. 1992 discloses melt compounded thermoplastic compositions comprising Kynar® 720 polyvinylidene fluoride (PVDF) and a methacrylimide-containing polymer onto which an ethylenic monomer has been grafted. The said compositions retain the excellent heat resistance, weatherability, mechanical properties, and moldability inherently possessed by the methacrylimide-containing polymer, and posses as well the characteristics of the PVDF. The preferred degree of imidization is ranging from 50 to 95%.
Blends of matrix polymers comprising Kynar® 740 polyvinylidene fluoride (PVDF), compatibilized with imide polymers, are described by the EP 0438239 A (ROHM AND HAASCOMPANY) 11 Jan. 1991 application. The preferred degree of imidization is ranging from 40 to 100% and the higher the degree of imidization, the greater the effect on properties such as modulus and heat distortion.
It has been shown in EP 0438240 A (ROHM & HAAS [US]) 16 Jan. 1990 that block copolymers of olefin and imide polymers are useful as compatibilizers in blends with thermoplastic polymers (e.g. PVDF, styrene/acrylonitrile . . . ). No indication has been provided about the degree of imidization.
EP 0570135 A (ROHM & HAAS [US]) 13 May 1992 describes impact modifier polymers having rubbery cores and multiple hard shells in which the said cores are protected from imidization and in which at least one of the hard shells comprises glutarimide, acid, anhydric and acrylate units. The resulting blends are useful in any of various applications in which high heat resistance and impact strength are required. These polymers might be used in combination with PVDF by melt compounding. Some examples are shown whereby the degree of imidization is ranging from 43 to 62%.
The use of polymethacrylimide polymers in a thermoplastic admixture with PVDF as adhesion binder and simultaneously, barrier materials towards fuel has been shown in U.S. Pat. No. 5,795,939 (ATOCHEM ELF SA [FR]) 28 Apr. 1994. No indication has been provided about the degree of imidization.
U.S. Pat. No. 5,510,160 (HUELS CHEMISCHE WERKE AG) 28 Jan. 1994 and U.S. Pat. No. 5,554,426 (HUELS CHEMISCHE WERKE AG) 24 May 1995 describe both the use of polymethacrylimide polymers in an admixture with PVDF as an adhesive bonding layer in a thermoplastic multilayer composite comprising a polyamide layer. These adhesive bonding layers has been claimed to provide improved resistance to chemical agents.
Nevertheless, certain blends fail to provide thermodynamically miscible compositions, so that their optical properties can be dramatically affected. Also, none of the prior art document provides for a suitable solution to the improvement of hardness/abrasion compromise in VDF polymer-coating compositions.
There is thus still a need in the art to develop novel coating compositions based on vinylidene fluoride polymers and polyacrylimide polymers, said composition providing improved hardness and abrasion resistance while maintaining adhesion, optical and other coating properties typical of highly weatherable coatings.